Send email from a subdomain (notifications.acme.com, news.acme.com) rather than your root domain (acme.com). It costs a few extra DNS records and buys you two things you’ll be very glad to have on a bad day.
Reputation isolation
Things happen. Someone floods your signup form and you fire tens of thousands of verification emails at burner addresses. A campaign gets marked as spam at scale. When that damage lands on a subdomain, you can quarantine it—pause the stream, fix the problem, even retire the subdomain—while the rest of your email keeps flowing. When it lands on your root domain, everything you send wears the damage, and root-domain reputation is a long road back.
Purpose transparency
Not all mail deserves the same inbox treatment, and providers know it. A password reset is urgent; a monthly product digest is not. Gmail and Outlook constantly triage incoming mail into priority, promotions, and spam—and consistent, purpose-specific subdomains give them a clean signal for what each stream is. That predictability builds trust. A sensible split:
send.acme.com—transactional (resets, receipts, confirmations)news.acme.com—marketing and newsletters
One hard rule: no lookalike domains
Whatever you do, don’t “protect” your root domain by buying getacme-mail.com or acme-alerts.com. Brand-adjacent lookalikes are the shape of phishing: spam filters treat them with suspicion, and your recipients—trained for years to distrust almost-your-brand domains—will ignore or report them. Subdomains of the real domain give you the same isolation with none of the suspicion, and they inherit your brand’s recognition instead of diluting it.
The honest caveat
If you’re sending a trickle of transactional mail and nothing else, root-domain sending won’t sink you. But domains are habits: set up the subdomain on day one and you’ll never have to migrate reputation later. It’s the cheapest insurance in your DNS zone.